This invention relates to a brake unit for automotive vehicles, with brake slip and traction slip control, and being equipped with a power-assisted brake pressure generating assembly with which the wheel brakes are connected through hydraulic fluid lines. The brake unit includes an auxiliary hydraulic pressure supply system comprising a hydraulic pump, an auxiliary pressure modulation valve controllable by the brake pressure generated in the brake pressure generating assembly, and a pressure balancing tank. In addition, valve arrangements are provided which are inserted in the hydraulic fluid lines leading to the wheel brakes succeeding the brake pressure generating assembly. The valve arrangements are switched to free passage in their normal position and upon commutation into a second switching position, link the auxiliary pressure supply system, in lieu of the brake pressure generating assembly, hydraulically to the wheel brakes. The brake unit is, furthermore, provided with wheel sensors and with electronic switching circuits for detection and monitoring of the rotational behaviour of the wheels and for the generation of electric brake pressure control signals by means of which electromagnetically actuatable brake pressure modulators are controllable for the purpose of slip control.
In German patent application No. P 35 02 451.8 a brake unit of this type is described which in one embodiment is comprised of a master cylinder with preceding vacuum booster. If and when locking of a wheel is imminent, a pressure is built up in the auxiliary hydraulic pressure supply system whose level is controllable, with the aid of the auxiliary pressure modulation valve, through the pressure existing in the power chamber of the master cylinder and being proportional to the pedal force. The auxiliary pressure leads to the commutation of multiway valves which in their normal position connect the power chambers of the master cylinder with the wheel brakes and upon commutation link the wheel brakes to the controlled auxiliary pressure instead of to the master cylinder. The brake pressure modulators controlled by the slip control are inserted in the hydraulic fluid paths between the described multiway valves and the wheel brakes, respectively between the wheel brakes and the balancing tank. It will be appreciated that in this manner the brake circuits of that brake unit are static in the event of non-controlled braking operation, whereas the brake pressure is supplied dynamically from the auxiliary pressure supply system on the start of the slip control action and upon the commutation of the multiway valves. When the control action starts, the exits of the master cylinder are shut off. It is ensured even in unfavorable conditions, in particular in the event of a strong pressure drop when driving on a slippery road surface as well as in the event of a failure of the auxiliary pressure supply, that sufficient hydraulic fluid remains at disposal in the master cylinder and, consequently, in the brake circuits.
The object of the present invention is to further develop a brake unit of this kind with a minimum of design efforts in such a manner that the brake unit is not only suited for the control of the brake slip but also for the control of the traction slip.